1. Field of the Invention
The present invention relates to an active-matrix liquid crystal display (LCD) in which liquid crystal display is performed by driving each pixel by a thin film transistor (TFT) and, more particularly, to a liquid crystal display (LCD) using for a thin film transistor poly silicon made through a low-temperature process.
2. Description of the Prior Art
A liquid crystal display which seals in liquid crystal between a pair of substrates and applies voltage to the liquid crystal to perform a desired display has advantages of compactness, thinness, and easy reduction of power consumption. Such displays are therefore put to practical use in various office-automation, audio-visual, and portable or on-board information devices. Especially, active-matrix liquid crystal displays using thin film transistors (TFTs) as a switching device for driving each liquid crystal pixel can display higher-definition images without cross talk because liquid crystal pixels can be selectively driven.
A noncrystal silicon TFT using noncrystal (amorphous) silicon in its active layer and a poly silicon TFT using poly silicon of higher mobility in its active layer are both used in liquid crystal displays. Noncrystal silicon TFTs are often used in large-sized displays or the like because they can be formed in a wide area through a low-temperature process. On the other hand, the mobility of poly silicon is higher than that of noncrystal silicon and poly silicon can form a device by self alignment. As it is therefore easy to make the area of a TFT and a pixel with a poly silicon TFT smaller than that of a TFT and a pixel with a noncrystal silicon TFT, a poly silicon TFT is often preferred for use in high-resolution displays. Moreover, if poly silicon is used, it is easy to make a TFT CMOS structure. As a result, a display area TFT and a driver TFT for driving it can be formed on the same substrate through almost the same process.
As stated above, a poly silicon TFT has attractive characteristics and can have a driver on its substrate. Furthermore, poly silicon is known to be formed by polycrystallizing noncrystal silicon through a high-temperature (above 600.degree. C. ) process. In this case, noncrystal silicon is exposed to a high-temperature during a process, which makes it impossible to use a cheap glass substrate. A difficulty therefore arises when a poly silicon TFT is put to practical use.
Polycrystallizing technology using annealing treatment such as laser or lamp annealing has, however, been improved to the point where it is now possible to make poly silicon through the so-called low-temperature (below 600.degree. C. ) process. This method of forming a poly silicon TFT through a low-temperature makes it possible to use a cheap glass substrate and thereby reduce cost. Furthermore, this method makes it possible to form poly silicon TFTs in a wide area. As a result, poly silicon TFTs formed through a low-temperature process (hereinafter referred to as a low-temperature poly silicon TFT) have been put to practical use.
Although low-temperature poly silicon TFTs have been put to practical use, the most suitable liquid crystal materials and panel configuration for demonstrating the characteristics of a low-temperature poly silicon TFT as a liquid crystal display and for improving its characteristics have not been developed yet. As a result, materials and a configuration, for example, used in a liquid crystal display using a conventional noncrystal silicon TFT are directly adapted, which means that the resulting poly silicon TFTs are not able to fully demonstrate their unique characteristics.